26 August 2009

Were glyptodonts’ clubbed tails weapons?

Yesterday, I wrote about ankylosaurs’ clubbed tails. Today, I get another new paper on another group of vertebrates to have clubbed tails, the massive armored mammals called glyptodonts. To the best of my knowledge, these two groups may be the only vertebrates to have massive bony clubs on their tails. This paper is also concerned with whether glypotodonts could use their tails as weapons, but takes a decidedly different approach.

Blanco and company are trying to characterize a feature in the glypotodonts’ clubbed tail that we are familiar with in our own clubs: the center of percussion. Annoyingly, the authors don’t define this, or explain why it is interesting, in their introduction.

The center of percussion is a “butter zone” or “sweet spot.” If you wield a club – say, a baseball bat (which the authors use in their figures) or cricket bat – the center of percussion is the point where hitting something has the least effect on the closer joints. Hit away from that point, and the force is transmitted through rest of the structure. If you hit a ball too close to your hands, you’ll feel it more, and the flow of your movement is interrupted.

How this relates to the “club as weapon” hypothesis is that glyptodonts’ clubs may have had thick spikes or pads on the club itself. If they were there, they were not bony, so didn’t preserve, but the structure of the clubs is suggestive. If the tails were being used as clubs, you would predict that the center of percussion would be right in the center of the club.

Blanco and company worked with the fossils of five different glyptodont species, whose clubs ranged in mass from an estimated 2 kg to a whopping (pun intended) 50 kg or so. While some glyptodonts had tails that were flexible,in these five, some of the bits at the end of the tail are completely fused, making it more like a true, rigid club.

They measured the sizes of the tail bones, estimated their mass and density, and ran the numbers to estimate the location of the center of percussion. They varied their estimates of the bone density, but this didn’t move the predicted center of percussion much at all. The authors found that the center of percussion was pretty much square in the center of the clubbed end of the tail, as predicted, nicely sitting in among where those spikes or pads are thought to be.

This paper has an hypothesis in common with the ankylosaur paper in that it suggests the club was being used, not as a defense against predators, but as a weapon in fights with other glyptodonts. I am skeptical, because when you look at animals that fight within other members of their species, the weapons are at the front end: crayfish claws, mountain sheep horns, elephant tusks... The list goes on and on.

Despite my skepticism, there seems to be fossil evidence of the sort I was talking about yesterday that support it: broken bones and wounds in glyptodont fossils that could have been caused by another glyptodont. But why glyptodonts and ankylosaurs alone should have stuck their nasty bits at the rear end is a strange evolutionary puzzle.